(1) Field of the Invention
The present invention relates to split-gate flash memory cells, and in particular, to a high density multi-bit split gate (MSG) flash EEPROM where bit by bit erasing is performed in order to enhance the bit alterability.
(2) Description of the Related Art
Flash EEPROM products combine the fast programming capability and high density of erasable programmable read only memories (EPROMs) with the electrical erasability of EEPROMs. As is well known, all flash EEPROM products are based on the floating gate concept. The memory can be erased electrically but not selectively. The content of the whole memory chip is always cleared in one step. The advantages over the EPROM are the faster (electrical) erasure and the in-circuit programmability, which leads to a cost-effective package.
A flash memory device usually includes an array of EEPROM cells in rows and columns, along with addressing decoders, sense amplifiers and other peripheral circuits necessary to operate the array. In addition to the charge on a floating gate affecting the conduction between source and drain regions of the individual memory cells, a control gate which extends across a row of such cells to form a memory word line also controls the floating gate potential through a capacitive coupling with the floating gate. The source and drain regions form the memory array bit lines. The state of each memory cell is altered by controlling the amount of electron charge on its floating gate. One or more cells are usually programmed at one dine by applying proper voltages to their control gates, sources and drains to cause electrons to be injected onto the floating gates. Prior to such programming, a block (sector) or such cells is generally erased to a base level by removing electrons from their floating gates to an erase electrode. In one form of device, this erase electrode is the source region of the cells. In another form of the device, a separate erase gate is provided.
Various techniques are being used in the semiconductor industry to increase the storage density of flash EEPROM memories. As is occurring with integrated cu-cults generally, the sizes of individual circuit elements are being shrunk as processing technology improves. In addition, flash EEPROM memory cells are being designed to store more than one bit of data by establishing multiple charge storing states for each cell. The effect of these trends is to shrink the size of the memory blocks (sectors) which store a set amount of data.
One such technique is to share the source and drain regions interchangeably between adjacent cells on the same word line of a split-gate flash memory. For example, in the dual split-gate (DSG) shown in FIG. 1a, two floating gates of the cells (A) and (B) share the same source/drain(S/D). More specifically, and as seen more clearly in the cross-sectional view (1b) of the same substrate (10), the memory cell is a triple polysilicon split-gate structure in which the floating gate (30) above gate-oxide (20) is polysilicon level 1, control gate (50), separated from the floating gate by inter-gate oxide (40), is polysilicon level 2, and the word select gate (80), separated from the control gate by nitride layer. (60) is polysilicon level 3. It will be noted that third polysilicon (80) is isolated from both the floating gate and control gate by oxide spacer (70) as shown in the same Figure. Source/drain diffusions (13) are placed every two floating gates apart, thus improving density over the conventional cell, which has separated source and drain regions. Although two floating gates share the same word gate, source and drain regions, read and/or program to a single floating gate is possible because control gates are separated. Above each of the floating gates lies a control gate which controls the voltage of the individual floating gate by capacitance coupling. The control lines run parallel to the source/drain. Some of the disadvantages of the DSG cell are high program voltages of about 12V and also high voltages during read. A high control gate voltage of 12V is required during read operation when one of the floating gates is being accessed in order to mask out the effects from the other floating gate. Adjacent cells which may share the same diffusion or control gate voltages will be effectively disabled from the operation by suppressing the other floating gates with a very low ˜0 control gate voltage. The same kind of over-ride and suppress techniques are used during program in order to target a single floating gate cell. In this way, program and read operations can be performed on the high density, self-aligned dual-bit split-gate flash/EEPROM cell.
As described more in detail by Y. Ma, et al., in U.S. Pat. No. 5,278,439 the DSG shown in FIGS. 1a and 1b contains two bits, A and B, one in each cell. This can be better understood by considering FIGS. 1d and 1e with the key shown in FIG. 1c. (See also, Y. Ma paper on “A Dual-bit split-Gate EEPROM (DSG) Cell in Contactless Array for single-Vcc Height Density Flash Memories”). The cell has two floating gates, one control-gate (CG), one transfer-gate (TG), one common selected gate (SG), and the two bits share one pair of drain (D) and source (S). As shown in FIG. 1b, the CG and TG are structurally identical. The SG channel is formed by a split-gate located between CG and TG. The dual-bit cell is accessed by five terminals, as shown in FIGS. 1d and 1e. The conventions of the five active terminals are referred to as the left (90) or right (95) selected bit in the cell, as indicated in the same Figures. It will be apparent to those skilled in the art that in comparison with a conventional single-bit cell, the DSG's cell size savings comes directly from the shared and self-aligned SG. Of the three directly connected channels (CG, SG, and TG) between the source and drain, two work as a transfer channel (17), one as control-channel (15) for the selected channel, as shown in FIG. 1b. During an address switch between the left and right bits in the cell, the CG and TG terminals exchange their functions, so do the terminals of drain and source. Within a cell, the two bits are reciprocally equal.
The various program (write), erase and read operations for a DSG are illustrated in FIGS. 2a-2c and 3a-3c. FIGS. 2a-2c schematically represent the cross-sectional views of a DSG while FIGS. 3a-3c represent a top view of the same DSG where Figure numbers with the suffixes (a), (b) and (c) refer to the write, erase and read operations, respectively. The key shown in FIG. 1c also apply to FIGS. 2a-2c and 3a-3c so that the five terminals shown in FIGS. 2a-2c would be impressed with voltage (V) appropriate to the particular gate corresponding to each one of the operations.
Thus, keeping the same reference numerals in FIGS. 1a-1e referring to similar parts throughout the several views in FIGS. 2a-2c and 3a-3c, FIGS. 2a and 3a show the program or write operation for the same DSG as before. The write operation is performed bit by bit and the programmed bit is selected by a selected gate or word line (80) and bit line (13). In the write operation, source-side-injection mechanism is used where the selected gate (SG) is only weakly turned on so as to just turn on channel of unselected cell (30u) while a higher voltage is used on control gate (CG) to provide higher vertical electric field to complete the write operation. In other words, hot-electrons (12W) are created at the transitional channel region (17) between SG and CG, and injected to the source side of the floating gate (30s) while TG and CG are strongly turned on. The various voltage levels are shown for the program operation in both FIGS. 2a and 3a. 
In the erase operation shown in FIGS. 2b and 3b, negative-gate Fowler-Nordheim tunneling is used. Thus, during erase, with negative voltage of −10V on CG, an applied drain voltage of 7V pulls the stored electrons out of floating gate (30s) via drain-side tunneling (12E), while the SG is grounded and the conduction channel cut off. As seen in FIG. 3b, erased bits (30s) are selected only by CG and a whole page of bits in the array are erased.
The read operation is accomplished by selecting the read bit by word line (80) and bit line (13) as in the write operation except that the TG and SG are fully turned on and the stored information is sensed by detecting whether the is channel current (12R) under the grounded CG.
In prior art there are other schemes for forming triple polysilicon flash EEPROM arrays with dual-bit capabilities. In U.S. Pat. Nos. 6,028,336 and 5,712,179 by Yuan, a triple polysilicon flash EEPROM array having a separate erase gate for each row of floating gates, and methods of manufacturing such an array are disclosed. As part of a flash EEPROM array on a semiconductor substrate, erase gates are formed in individual trenches between rows of floating gates. The erase gate is positioned along one sidewall of the trench in a manner to be capacitively coupled with the floating gates of one of the rows adjacent the trench but spaced apart from the floating gates of the other row adjacent the trench. In this way, a separate erase gate is provided for each row of floating gates without increasing the size of the array. The erasure of each row is then Individually controlled. Two self-aligned methods of forming such an array are disclosed. One method involves forming a thick insulating layer along one sidewall of the trench and then filling a remaining space adjacent an opposite trench sidewall with polysilicon material forming an erase gate for the row of floating gates adjacent the other sidewall. A second method involves anisotropically etching a layer of polysilicon that is formed over the array in a manner to conform to the trench sidewalls, thereby separating the polysilicon layer into individual erase gates carried by the trench sidewalls.
In another dual floating gate EEPROM cell array, with steering gates that are shared by adjacent cells, E. Harari, et al., show in U.S. Pat. No. 6,151,248 how dual gate cells can increase the density of data stored. An EEPROM system has an array of memory cells that individually include two floating gates, bit line source and drain diffusions extending along columns, steering gates also extending along columns and select gates forming word lines along rows of floating gates. The dual gate cell increases the density of data that can be stored. Rather than providing a separate steering gate for each column of floating gates, an individual steering gate is shared by two adjacent columns of floating gates that have a diffusion between them. The steering gate is thus shared by two floating gates of different but adjacent memory cells. In one array embodiment, the floating gates are formed on the surface of the substrate. In arrays that erase the floating gates to the select gates, rather than to the substrate, the wider steering gates uncouple the diffusions they cover from the select gates. This use of a single steering gate for two floating gates also allows the floating gates, in another embodiment, to be formed on side walls of trenches in the substrate with the common steering gate between them, to further increase the density of data that can be stored. Multiple bits of data are also stored on each floating gate.
Low voltage erase of a flash EEPROM system having a common erase electrode for two individual erasable sectors are shown in U.S. Pat. No. 5,677,872 by G. Samachisa, et al. Here also a flash EEPROM is organized on an integrated circuit with individual erase gates being shared by two adjacent blocks, or sectors, of memory cells. This is to reduce the number of erase gates and the complexity of the driving erase circuitry. Also, according to Guterman, et al., U.S. Pat. No. 6,222,762 teaches maximized multi-state compaction and more tolerance in memory state behavior through a flexible, self-consistent and self-adapting mode of detection, covering a wide dynamic range.
As useful as dual-bit split-gates (DSG) and multi-state memory cells are, further improvements can be achieved by multi-sharing of source/drain regions in the manner disclosed below in the embodiments of the present invention. Also, erasing function, in general, can be improved as shown below.